In the case of modern braking systems for commercial vehicles, electrically controllable electro-pneumatic modulators (pressure control modules) are assigned to the individual wheel brakes. These modulators control the brake pressure into the brake cylinders corresponding to the driver""s braking demand. The pressure control modules each have an electro-magnetic switching valve, which switches in a pulse-type manner and operates to allow storage air into the control chamber of a relay part to vent it.
For a brake pressure control which is as exact as possible, the same pulse times in every operating condition of the brake system, if possible, should cause the same brake pressure changes. In particular, independently of the momentary supply voltage, the same current flow should always exist in the electromagnet of the switching valve during a pulse. The reason is that it is endeavored to provide a brake pressure control system which can be used worldwide and by which different supply voltages between 9V and 36V can be controlled. For a sufficiently large xe2x80x9cmagnetic switching forcexe2x80x9d, the magnet coils, while the supply voltages are low, must have low ohmic resistances. However, at high supply voltages, low resistances would result in very large coil currents.
In the case of pressure control modules, high xe2x80x9cswitching dynamicsxe2x80x9d are also required, for which the transmission cross-sections should be relatively large. However, in order to simultaneously achieve a good gradualness of the brake pressure, very short opening times should be possible. However, the valves must open up rapidly at the beginning of a pulse and drop again rapidly at the end so that a defined armature movement can still be accomplished with such short opening times. This has the result that first a large amount of energy, that is, a high magnetic flux, must flow very rapidly into the coil so that the armature lifts rapidly and reliably off the valve seat. At the pulse end, the energy stored in the coil should be reduced as fast as possible so that the armature can rapidly be moved back again by a restoring spring in order to seal-off the valve seat. This requires a magnet current which is as low as possible. Despite the required high switching dynamics, the magnets should have a high-percentage switch-on duration (ED), also at operating temperatures of approximately 80xc2x0 C. This requires low currents.
It is an object of the invention to provide a system for controlling the brake pressure which meets the above-mentioned requirements.
This object is achieved by a system for controlling brake pressure valves which have a storage pressure input, a brake pressure output, an electromagnet and an armature. The armature can be operated by exciting the electromagnet, for opening-up or shutting-off the storage pressure input with respect to the brake pressure output. The system includes a voltage source and an electronic control system for pulse-type excitation of the electromagnet. A measuring device is included for determining the actually present supply voltage. The electronic control system in order to generate a defined opening or closing movement sequence of the armature, generates voltage pulses for exciting the electromagnet and varies the pulse times as a function of the actually existing supply voltage. Advantageous developments and further developments of the invention are described herein.
The basic principle of the invention consists of the fact that, during operation, the supply-voltage available for controlling the electromagnet of the brake valve is continuously measured and the active value of the voltage present at the coil of the electromagnet is varied by the variation of the xe2x80x9cpulse duty factorxe2x80x9d, that is, the pulse times as a function of the actually present supply voltage.
In other words, for generating a desired opening or closing sequence of the armature, the pulse duty factor is adjusted by an electronic control system as a function of the momentary supply voltage. The variation of the pulse duty factor can be implemented by the variation of the xe2x80x9cwidthxe2x80x9d, that is, the duration of the voltage pulses, and/or by the variation of the duration of the non-excited states situated in-between.
In this case, the magnet is designed such that, also at the lowest possible supply voltage occurring during the operation, the magnet still attracts reliably and rapidly; that is, that a defined minimum attraction current IAmin is clearly exceeded.
By using an electronic control system, the active value of the control voltage of the electromagnet is controlled by the adaptation of the excitation timing such that, at the start of the pulse, a defined attraction voltage UA is always present at the electromagnet. For this purpose, the pulse duty factor is correspondingly adjusted as a function of the momentary supply voltage UV.
After a certain attraction time, the armature has securely been pulled-in completely. In order to hold the armature in the operated position, a much lower holding current IH and thus also a lower active voltage is required which, in the following, will be called a holding voltage UH. After the attraction voltage UA has been present at the electromagnet for a defined time period, the pulse duty factor is adjusted as a function of the supply voltage UV such that the holding voltage UH is set. As a result, the magnetic current IH, and thus the energy stored in the magnet coil, will fall, whereby the percentage of the switch-on time duration is increased. The now lower energy can be reduced more rapidly at the end of the pulse, which improves the switching dynamics.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.